In response to the growing need to handle high temperature reactive metals and alloys in today's manufacturing industry, various non-contact materials processing techniques have been developed. For example, a typical maximum temperature in a gas turbine may be on the order of 2000.degree. F. The alloys developed for use at these temperatures all melt at extremely high temperatures. New alloys that are under investigation have melting points approaching and exceeding that of high temperature refractory ceramics, making them difficult to melt, cast, and forge. For instance, the tungsten and molybdenum alloys have melting points above the softening points of the highest temperature refractories. Using a conventional crucible to contain and melt these alloys would result in contamination of the specimen by material from the crucible itself.
One solution to this problem is to leave a layer, called a skull, of solid metal between the crucible and the metal being melted. In addition, vacuum melting is commonly used in high temperature applications to melt higher percentages of reactive metals, improve mechanical properties (including fatigue strength, ductility, and impact strength), decrease scatter in the mechanical properties, and improve the billet to bar stock conversion ratio in wrought alloys.
A non-contact casting process would allow melting and casting of even the highest melting temperature metals without contamination from the crucible and without needing a skull layer to separate the molten metal and the crucible. In this manner, levitation casting would also allow a bar of alloy to be zone-refined. In zone refining, a slice, or zone, of the bar is melted, and this molten zone is moved along the length of the bar. Impurities are trapped in the molten zone, and eventually concentrated at one end of the bar, which is then removed. The zone refining process can be repeated on a single specimen to yield ultra-pure metals, and is commonly used in the semiconductor industry to purify silicon crystals. Non-contact levitation of the molten zone during this process would allow the molten zone to be contained without contamination from the container.
One non-contact levitation technique uses alternating currents in a coiled conductor to create time-varying magnetic fields that can be used to contain nonmagnetic conducting materials. With respect of their magnetic properties, materials may be grouped into three categories: ferromagnetic, paramagnetic, and diamagnetic. Ferromagnetic materials can support long range ordering of their magnetic moments, and thus have a relative permeability much greater than one. Relative permeability is a measure of the ease with which magnetic fields can be established in a material, as compared to vacuum. Permeability is the magnetic analog of electrical conductivity; high conductivity material passes electricity readily, high permeability aterial passes magnetic fields readily. Paramagnetic materials have a relative permeability on the order of 1.00001, arising from the magnetic dipole moment of their spinning electrons. Diamagnetic materials have a relative permeability of 0.99999, arising mainly from the orbital motion of the electrons within their atoms.
Materials with a relative permeability greater than one will experience a force oriented so as to increase the magnetic field within the material when exposed to static or dynamic magnetic fields. Materials that produce their own magnetic field will also experience a force orienting their field parallel to the external field. Materials with a relative permeability less than one will experience forces oriented so as to reduce the magnetic field strength within the material. For all practical purposes, both paramagnetic and diamagnetic materials may be assumed to have a relative permeability of one. i.e.; they are not affected by static magnetic fields. Some ferromagnetic materials, such as iron, change their relative permeability with temperature. Above a certain temperature, referred to as the curie point, iron becomes non-magnetic. This state can also be produced at room temperature by the addition of certain alloying elements, as in the 300 series of stainless steels.
In addition, dynamic magnetic fields and/or moving materials will induce eddy currents in conductive materials that act to reduce the magnetic field in the material. The eddy currents produce ohmic heating in the material, and are acted upon by the magnetic field, producing a force on the conducting material called the Lorentz force. These forces are related to the gradient (change in magnitude versus change in position: slope) of the magnetic field and are directed from areas of high gradient to areas of low gradient. Under certain conditions, the Lorentz forces and heating can be relatively controlled so as to produce levitation and/or melting of a conducting body. Electromagnetic levitation melting is known in the art and various levitators have been designed for the purpose of containing and levitating solid and molten metal."
One such levitator is disclosed in Electromagnetic Levitation Melting of Large Conduction Loads, Sagardia et al., IEEE Transactions on Industry Applications, vol. 1A-13, No. 1, January/February 1977. The Sagardia levitator has a vertical axis and contains the specimen in a generally bowl-shaped field. In order to prevent the liquid (molten) sample from leaking out through the bottom of the levitator, Sagardia uses multiple coils wound around different axes and operating at different frequencies. Sagardia's method produces a high rate of heating relative to the levitation force.
Other levitators use one or more helical coils wound into a generally conical or concave shape with the axis of the helix vertical carrying current of a single frequency. These single frequency levitators all have an area on the bottom of the levitated specimen where the levitation force is zero. At this point, all that opposes the downward pressure of the molten metal is surface tension. This pressure depends on the height of the specimen which, in combination with the shape produced by the eddy currents elsewhere, is also related to the volume of the specimen. When the height of molten metal exceeds a critical value, it leaks out of the force-free region. Given a generally top-shaped specimen, this leakage places a severe restriction on the mass of metal that can be levitated by this method. Other disadvantages of known levitators include: lack of control over specimen position within the levitator, limitations on the shape of the specimen, and lack of visual or physical access to the specimen."
U.S. Pat. No. 4,414,285 discloses a non-contact vessel for containing a stream of molten metal. The device requires a flow of molten metal under pressure up into the lower end of the vessel and a cooling zone adjacent the upper end of the vessel for receiving the cast metal. The '285 device works by applying an upward force to metal in the non-contact zone, thereby eliminating hydrostatic head from the portion of metal immediately below it and allowing the pressurized metal to flow up into the non-contact zone. The non-contact zone provides an upward force by operation of a plurality of circumferential coils, each operating 60 degrees out of phase with its neighbors, forming a linear polyphase motor. When alternating current is passed though the '285 device, it forms a traveling magnetic field inside the coils. This traveling field pulls the molten metal upwards, acting in a fashion similar to that of the rotating field in a normal polyphase motor. The '285 device requires the use of a "starting rod" that is joined in the initial stage of the process to the liquid metal column for the purpose of lifting the metal column into the device to initiate feed. The '285 device also requires an external cooling medium.
It is desired to provide an electromagnetic levitator in which the specimen heating associated with the application of a given levitation force is minimized. As heat can always be supplied by an external source, such as a second coil, minimization of specimen heating by the levitator allows separation of heating and levitation and therefore allows greater control over the process. The heat generated in a specimen per time and volume unit is proportional to the magnetic flux density, while the force exerted on a specimen per volume unit is proportional to the gradient of the magnetic flux density. Thus, the objective of minimizing heating while maximizing levitation may be met by providing an electromagnetic levitator that includes an operating area wherein the magnetic flux density is relatively small but subject to a steep gradient.
It is a further object of this invention to provide a levitator that is capable of levitating a specimen having a mass greater than the mass of specimens levitatable by previously known levitators. It is a still further object of this invention to provide a levitator that allows the specimen to be easily viewed and/or manipulated during levitation and maintains the specimen in a stable levitation zone. Other objects and advantages of the invention will appear from the following description.